Radio navigation system

ABSTRACT

Guidance signals are provided for trips between two points by two pairs of radio signals transmitted from fixed locations. The first of these two pairs of signals which are used for making distance measurements are transmitted from locations essentially ahead of and behind the track between the starting and finishing points of the trip to be made. A controller on the vehicle is calibrated so that a reading is provided at the start of the journey in accordance with the change in relative travel times the two signals will experience from the start to the finish of the journey, this reading being indicated as the distance to be traversed during the trip. As the distance to the destination is reduced, the distance readout is reduced accordingly. The second pair of stations, which are preferably located along a line substantially normal to and on either side of the vehicle track, provide signals which are utilized in conjunction with the distance signals to provide a left ro right steering signal whenever the vehicle departs from the desired track, a calibration reading being set into the controller in accordance with the change in relative travel times the second pair of signals will experience from the start to the finish of the trip.

States e [1 1 Wipif et al.

[451 Aug. 28, 1973 RADIO NAVIGATION SYSTEM [75] Inventors: Frank P.Wlpfi; James A. Wilson, [57] ABSTRACT both of Prescott, Afil- Guidancesignals are provided for trips between two [73] Assignee: AirborneNavigation Corporation, points by two pairs of rad1o signalstransmlttedfrorn Prescott, Adz. fixed locatlons. The first of these twopairs of signals which are used for making distance measurements areFiled: y 1971 transmitted from locations essentially ahead of and be- 21A L N z 145 796 hind the track between the starting and finishing points1 pp 0 of the trip to be made. A controller on the vehicle is calibratedso that a reading is provided at the start of 343/112 1 343/103, 343/ 112 the journey in accordance with the change in relative 235/1502,235/150-27 travel times the two signals will experience from the [5 l 1ll!- Clstart to the finish of the journey this reading being indi- Fieldof Search cated as the distance to be traversed during the AS 343/103,105 1 12 112 1 12 R the distance to the destination is reduced, thedistance readout is reduced accordingly. The second pair of sta- [56]References cued tions, which are preferably located along a linesubstan- UNITED STATES PATENTS tially normal to and on either side ofthe vehicle track, 2,472,129 6/1949 Streeter 343/112 c Provide Signalswhich are utilized in Conjunction with 2,844,816 7 1958 O'Brien et a1343 R the distance signals to provide a left to right steering 2,582,5881/1952 Fennessy et al 343/112 C signal whenever the vehicle departs fromthe desired 3,095,567 6/1963 Britnell 343/1 12 C track, a calibrationreading being set into the controller Bridges R r in accordance thechange in relative travel times the second pair of signals willexperience from the start Primary Examiner-Benjamin A. Borchelt to thefi i h f the trip Assistant Examiner-Denis H. McCabe .5 1. aAttorney-Sokolski & Wohlgemuth 16 Chums, 15 Drawmg Flgures TRANSMITTER 1gramme? 2 TRANSMITTER B" 14 if I I E "A" -TRANSMITTER PATENTEDMIB28 I9753 7 55, 817

SHEET 1 UF 8 TRANSMITTER INV EN TORS FRANK F! WIPFF JAMES A. WILSON BYSXOLSKI a WOHLGE MUTH ATTGQNEYS PMENIEMucza ms SHEET 8 BF 8 INVENTORSFRANK F? WIPFF JAMES A. WILSON Ami FQQV SONILSKI Bu WOHLGEMUTH ATTORNEYSmmmmzs ms 3.755;a1 7

SHEET 7 BF 8 10 L TRANSMITTER ORIGIN *3 v ossnmmorfiz o\ f o y I 3 a T 1ORIGIN v. DESUNATW n T K omsm 2 E DESTINA'HON 3 T0 R i Fl 6 l 1TRANSMITTE WAVE PE MW) E V V E WAVE SHAPE C(GND) AT (B) FIG. 9A

INVENTORS FRANK P WIPFF JAMES A. WILSON SO OLSKI 8s WOHLGEMUTH ATTORNEYSPATENTEB AUG 2 8 I873 SHEET 8 0F 8 DIST. FF AT T m mssus FIG. IOA

L-R FF AT T L-R FF AT FIG. IOC BY I B=TA=O TL TR 1 4 I TR I TR IN VENTORS FRANK F? WIPFF JAMES A. WILSON SDKOLSKI 8 WOHLGEMUTH ATTOR N EYSRADIO NAVIGATION SYSTEM This invention relates to radio navigationsystems and more particularly to such a system utilizing timingmeasurements between pairs of radio signals to determine the position ofa vehicle relative to points of origin and destination.

Radio navigation systems for guiding aircraft take several forms. Forrelatively short trips (under 1,000 miles) systems providing headingguidance signals, for example of the VCR type, are almost exclusivelyused. These types of systems, while accurate, are only available whenflying over well developed areas such as the continental United Stateswhere range stations are set up in elaborate networks between cities ofany significant size. However, when flying over less developed areas,radio guidance signals often are not available, leaving the pilotwithout this form of navigation.

To afford worldwide navigation capabilities, the OMEGA navigation systemutilizing very low frequencies, which with a small number of radiotransmitters is capable of covering the entire world, has beendeveloped. This type of navigation system, however, requires specialequipment adapted for use with the particular system in question. Suchequipment is generally of a very expensive and elaborate nature whichmust be carred on the vehicle. In view of the complexity of thisequipment, its maintenance and calibration for reliable operationpresents some problem. Each such system is separate in its operation andone must have specially designed equipment for use with each.

The system of this invention is an improved radio navigation systemwhich makes use of very low frequency signals such as provided by OMEGAtransmitting stations which have worldwide range, but does not use theOMEGA system or any other special system for its implementation butrather can use any radio signals of suitable related frequencies whichhave stable, reliable frequency characteristics and fixed and accuratelydetermined positions. Thus, the system of this invention can utilizesignals from communications transmitters at commercial and militarytransmitting sites, such as for example those used at worldwide Navalcommunication stations, as well as navigational transmitters.

The system of this invetnion operates to provide navigationalinformation for a trip between two predetermined points, thisinformation being in the form of a continuous indication of the distanceto the destination as well as steering information indicating deviationfrom a track line between the origin and the destination. While certainsystems of the prior art have been devised for providing this generaltype of information for a trip between an origin and a destination,utilizing phase measurements between radio signals in theirimplementations, these systems generally require directional antennas onthe vehicle in fixing the vehicles position and thus are somewhat morecomplicated and costly and less accurate than would be desired.

The system of this invention provides a relatively simple and economicalequipment which utilizes existing radio transmissions for making avoyage between a point of origin and a point of destination, this systembeing capable of utilizing signals from stations at considerabledistances from the vehicle, such that navigation over remote areas ispossible.

It is therefore an object of this invention to provide an improved radionavigation system.

It is another object of this invention to provide a radio navigationsystem capable of operation in remote areas.

It is still another object of this invention to provide an improvedradio navigation system capable of utilizing certain existingcommunications transmitters and other existing transmitters for itsimplementation.

It is still a further object of this invention to provide an improvedradio navigation system which can make use of transmitters located at agreat distance from the vehicle being controlled.

It is still a further object of this invention to provide a radionavigation system of great reliability and accuracy which is relativelyeconomical and simple in its construction.

Other objects of this invention will become apparent as the descriptionproceeds in connection with the accompanying drawings, of which:

FIG. 1 is a diagram illustrating the basic operation of the system ofthe invention;

FIG. 2 is a graphical illustration illustrating the computation of thedistance and steering information in the device of the invention;

FIG. 3 is a block diagram of the system of the invention;

FIG. 4 is a functional block diagram of one embodiment ofa receiver andfrequency divider which may be utilized in the system of the invention;

FIG. 5 is a functional block diagram illustrating the calibrationfunction of the system of the invention;

FIGS. 6A and 6B are a series of waveforms generated in the calibrationfunction of FIG. 5;

FIG. 7 is a functional block diagram illustrating distance measurementcircuitry which may be utilized in the system of the invention;

FIG. 8 is an illustration of waveforms developed in conjunction with thedistance measurement circuitry of FIG. 7;

FIG. 9 is a functional block diagram illustrating steering circuitrythat may be utilized in the system of the invention;

FIG. 9A illustrates waveforms generated in the circuit of FIG. 9;

FIGS. l0A-ll0C illustrate a series of waveforms which may be developedin the system of the invention in making various trips as shown in FIG.11; and

FIG. 11 is a schematic drawing illustrating the traversal of varioustrack lines with the system of the invention.

Briefly described, the system of the invention includes a group ofreceivers for receiving signals from several radio transmitters at fixedlocations. Two of these transmitters which are utilized for obtainingdistance information to a point of destination are located so that oneof the transmitters is essentially ahead of the vehicle while the otheris essentially behind the vehicle during the entire trip. The other pairof transmitting stations, which are utilized for providing steeringinformation to the destination point (one of which may be one of thetransmitters utilized for obtaining distance information), arepreferably located along a line substantially normal to the vehicletrack line from the origin to the destination. The equipment iscalibrated before leaving the point of origin so as to take into accountthe change in travel time difference to be expected between the signalsreceived from each of the two pairs of transmitters at the point oforigin and at the point of destination, the equipment calibration beingsuch that when the vehicle arrives at the point of destination, thedifference reading between the pair of distance signals is zero. Thesteering signals are compensated for by the distance signals, so thatwhen the vehicle is on the proper track, a zero" steering signal will bepresented, while when off the track to the left or right, appropriatesignals are provided to indicate such deviation. Thus, the time relatedsignals are utilized to provide distance information to the destinationas well as steering information to enable the steering of the vehicledirectly to such destination.

Referring now to FIGS. 1 and 2, the basic features of operation of thesystem of the invention are illustrated. Vehicle 11, onwhich theequipment of the invention is carried, is shown traversing a direct pathfrom an origin location to a destination location, D. Navigationalsignals for navigating the aircraft are transmitted from transmitter B,14, and transmitter A, 15, to provide continuous information as todistance to destination point D, while steering information indicatingany deviation from the direct path between the two points is provided bymeans of transmitter L 17, and transmitter R 18, which are to the leftand the right of the vehicle track respectively. It is to be noted thattransmitter A, 15, is ahead of the vehicle track, while transmitter B,14, is behind the vehicle track, the choice of transmitters in theserelative locations being necessary.

Referring to FIG. 1, let us designate the difference in the travel timebetween the signal transmitted to point 0" from transmitter B, and thesignal transmitted from this same transmitter to point D, T and thedifference in travel time between the signal transmitted fromtransmitter A to point 0 and that transmitted to D from this secondtransmitter T The change in the relative travel times and thus change intiming relationship between the signals received from these two stationsat point 0 as compared with point D. which results in a correspondingrelative timing change T between these two signals in travelling betweenthese two points can be represented as follows:

Referring now to FIG. 2, the relative timing change, T involved in goingfrom locations 0" to D is used as a calibration signal in an appropriatecomputer system, as to be explained further on in the specification, andif the aircraft is flown directly between points 0 and D," then thetiming change, T between the signals will be linear as indicated by lineT With the computer properly calibrated, the changing timing differencecan be made to be a linear reduction in T in traveling from point 0 topoint D.

Insofar as the signals received from transmitters 17 and 18 to the leftand right of the vehicle track respectively are concerned, a similarchange in timing difference between these two signals will occur intraveling between points 0 and D" for the illustrative example ofFIG. 1. Thus, the travel time of the signal arriving from transmitter 17to which aircraft 11 comes closer will decrease while the travel time ofthe signals arriving from transmitter 18 will increase. It is to benoted that if the track line 13 of the aircraft were normal to a linedrawn between transmitters l7 and 18, there would be no change inrelative travel time between the two signals in traversing the trackline between the two points. If aircraft 1] is maintained on track line13 in going between the points 0" and D and the difference in the traveltime of the signals is represented by T,- as shown in FIG. 2, then thisrelative travel time can be made to change linearly as indicated by lineT, during the trip. It should be immediately apparent that the followingrelationship exists:

The relationship set forth in equation (2) is implemented in the deviceof the invention to derive accurate steering signals in a highly uniquemanner, as to be explained more fully further on in the specification.It should be readily apparent from the foregoing discussion, that bymaking timing comparisons between signals arriving from transmittersahead of and behind the vehicle during the trip, that distanceinformation can be derived by virtue of the comparison between these twosignals, and that in similar fashion by making timing comparisonsbetween signals arriving from transmitters to the left and right of thetrack line, information can be derived in conjunction with the distanceinformation and a constant, which is indicative of any departure fromthe track line. It is to be noted at this point that one of theleft-right transmitters 17 and 18 can be one and the same as one of thedistance transmitters.

Referring now to FIG. 3, a block diagram illustrating the system of theinvention is shown. Signals transmitted from the four transmitters shownin FIG. 1 are received by antenna 19 and amplified in wide bandamplifier 20. Each of transmitters 14, 15, 17 and 18 transmits on adifferent frequency, the separate frequencies being indicated in FIG. 3as ,f ,f and f, being the outputs of transmitters A and B, while f and fare the outputs of transmitters L and R respectively. Receivers 2la-2ldare tuned to frequencies f,f respectively. The outputs of receivers2la-2ld, which it is to be noted are still at the originally receivedfrequencies, f,-f,, are each first clipped" to convert them to squarewaves, as to be explained further on in the specification, and then fedto an associated one of frequency dividers 23a32d. Each of the dividersdivides the frequency of the signal received thereby by a numbercorresponding to one-hundredth of this signal frequency.

Thus, with frequencies f,fl of the order of 12-25 kHz, all of thefrequency dividers have an output at a frequency of Hz, these signalshaving a phase relationship which changes in accordance with theposition of the vehicle relative to the various transmitters. The phaserelationship between these 100 Hz signals is the difference in the timeof occurrence of corresponding portions of these rectangular waves. The100 Hz rectangular wave outputs of frequency dividers 23a and 23b arefed to distance phase detector 24 which may comprise a flipflop, the setinput of which is triggered by the output of divider 23b and the resetinput of which is triggered by the output of divider 23a.

Phase detector 24 has a rectangular wave output, the positive goingportion of which has a pulse width corresponding to the phase differencebetween the outputs of frequency dividers 23a and 23b. In view of thefact that as already noted the phase difference between the two signalsreceived by the phase detector is indicative of the positionalrelationship of the vehicle with transmitters A and B, the pulse widthof the output of the phase detectors indicates this relationship. Theoutput of distance phase detector 24 is fed to digital distance computer25 which computes the distance travelled by the vehicle in response tothe relative phase information supplied by the phase detector. Theoutput of digital distance computer 25 is fed to distance readout 26,where a readout signal indicative of the distance travelled between thepoint of origin and destination is provided.

In order to provide information as to the distance travelled between theorigin and destination, distance computer 25 must be accuratelycalibrated prior to the start of the trip. As to be explained more fullyfurther on in the specification, calibration information is suppliedthrough calibration control 31, which includes the distance between thepoints of origin and destination and the change of relative phase to beexpected between the two signals as received at the origin, and asreceived at the destination. In implementing the calibration, frequencydivider 23a is reset by the calibration control to account for therelative phase change to be expected.

The outputs of frequency dividers 23c and 23d which, as already noted,are rectangular waves having a phase relationship which varies inaccordance with the position of the vehicle between the left and righttransmitters 17 and 18, are both fed to left-right phase detector 28which may be a flipflop operating in the same manner as described fordistance phase detector 24. The output of left-right phase detector 28thus is a rectangular wave having a positive going portion with a pulsewidth corresponding to the relative position of the vehicle between theleft and right transmitters. The output of phase detector 28 is fed tosteering computer 29. As to be explained in detail further on in thespecification in connection with FIG. 9, steering computer 29 receives acompensating signal from distance phase detector 24 and a calibrationsignal from calibration control 31 which in conjunction with theinformation received from left-right phase detector 28 enables thegeneration of a left-right steering signal for steering indicator 30.This steering signal may be in the form of a voltage which varies abouta zero point whenever the vehicle is off the desired track. Under suchconditions, steering indicator may comprise a voltmeter having a zero"center position.

Steering computer 29 must be calibrated before the trip is commenced bymeans of calibration control 31, the calibration information in thisinstance including the change to be expected in the travel time betweenthe signals driving from the left and right transmitters at the originand at the destination points and a fixed signal which represents abalanced steering condition desired when the vehicle reaches thedestination point. This calibration information is utilized as theconstant k" of equation (2) in implementing the invention.

Referring now to FIG. 4, a typical receiver and frequency divider whichmay be utilized in the system of the invention is schematicallyillustrated. It is to be noted that a separate receiver and divider asshown in FIG. 4 is provided for each of the separate signals, as shownin FIG. 3. It is also to be noted that in certain instances only threereceivers need be utilized, one of these receivers functioning dually toprovide one of the left-right and one of the distance signals. Thesignals received by antenna 19 are amplified in broad band amplifier 20and fed to radio frequency amplifier 40. where they are appropriatelyamplified. Amplifier is tuned to only one of the signal frequencies asshown in FIG. 4, this being the frequency,f which is the receiver A"signal of FIG. 3. Frequency f,, as already noted, typically may be ofthe order of 12-25 kHz.

Automatic gain control to optimize the operation of amplifier 40 isprovided by means of detector 41 and amplifier 42 which provides a gaincontrol signal to the amplifier 40 in accordance with the average outputthereof. The output of RF amplifier 40 is clipped by means of limitingamplifier 43, thus converting the sine wave signal to a square wavesignal. The square wave output of limiter 43 is fed to product detector44. The output of detector 44 is fed to integrating amplifier 45, andthence to voltage controlled oscillator 46.

Voltage controlled oscillator 46 is tuned to a substantially higherfrequency than f which may be 256 times this frequency, as shown in FIG.4. Voltage controlled oscillator 46 is phase locked with the 256thharmonic off by means of the phase lock loop which includes productdetector 44, integrating amplifier 45 and divider 48. This circuitoperates in the following manner:

The output of voltage controlled oscillator 46 is divided by 256 bymeans of divider 48, the output of this divider thus being at frequency,f Divider 48 provides a signal, A, which is in phase with the dividedoutput of the voltage controlled oscillator 46 and a signal, A, which is180 out of phase with this signal, to product detector 44. Productdetector 44 may comprise a phase detector which compares the phase ofthe output of limiter 43 with that of the output of divider 48, andprovides a DC output to integrator 45, the magnitude and polarity ofthis DC signal being indicative of the phase difference therebetween.This DC signal as integrated in integrator 45 is utilized to controlvoltage controlled oscillator 46 so as to phase lock its divided outputwith the input signal,f Phase lock circuits of the type just describedare well known in the art and are described, for example, in PHASE LOCKTECHNIQUES by Floyd Gardner, published by John Wiley & Sons.

The output of voltage controlled oscillator 46 is fed to divider 49wherein the signal is divided by one hundredth of the frequency of thevoltage controlled oscillator (I /100). Thus, the output of divider 49is a rectangular wave signal at 100 hz, this signal having a phasedependent on that off,. Divider 49 receives a reset signal fromcalibration control 31 (see FIG. 3), used in calibrating the system, asto be described in connection with FIGS. 5 and 6. A dead reckoningcontrol signal may also be provided to integrator 45 to afiord deadreckoning information where lapses in navigational signals occur.

Signals are also provided from divider 48 to product detector 50 forphase comparison with the output of limiter 43. Product detector 50produces a maximum output when oscillator 46 is phase locked with theinput signal. The output of product detector 50 is filtered andamplified in filter-amplifier 51 and the output of this amplifier usedas the supply voltage for indicator lamp 52. Thus, when the input signalis of sufficient strength and the voltage controlled oscillator is phaselocked to this signal, indicator lamp 52 will illuminate indicating thatthe signal is usable for navigation purposes.

Referring now to FIGS. 5, 6A and 6B, calibration circuitry which may beutilized in the system of the invention and waveforms developed inconjunction therewith are respectively illustrated. Momentarilyreferring to FIG. 1, it is first to be noted that prior to departurefrom the origin, suitable tables must be consulted to determined thechange in the difference between the travel times of the signalsreceived from the distance transmitters l4 and at the origin and at thedestination, and similarly the difference in travel time relationshipsbetween the signals from the steering transmitters 17 and 18 as receivedat the origin and as received at the destination. For convenience of usein the system, tables may be tabulated indicating in microseconds" thetravel times of signals from each of the transmitters to the origin anddestination, and then these tabulated values appropriately used tomanually compute the needed distance and steering calibrationinformation.

The distance calibration information is cranked into distance switch 52.For illustrative purposes, this distance information is shown in FIG. 5as 3456 this indicating the change in travel time difference between thesignals as received at the origin and as received at the destination.The calibration information for the left-right switch 53 is derived inthe same manner. However, in this instance 5000 is added to thedifference information, shown for exemplary purposes as 30, and thetotal 5030" cranked into left-right switch 53. Switches 52 and 53 maycomprise mechanically operated rotary switches which produce an outputin accordance with the reading set thereon, in binary coded decimalform.

Before describing the calibrate function, note that line 61 is normallylow by virtue of the fact that both inputs to NAND gate 56 are high. Toinitiate the calibrate function, calibrate switch 54 is momentarilyactuated. This produces a momentary negative going step, as indicated inline (C) of FIG. 6A. This results in a negative pulse being fed throughcapacitor 55 to one of the inputs of NAND gate 56. The negative pulse onthe input of NAND gate 56 forces the output of NAND Gate 56 (line 61) togo high, this high output being coupled through capacitor 58 andresistor 59 to the base of transistor 60. Transistor 60 is driven to aconductive state (saturation) by this voltage, thereby bringing itscollector to substantially ground potential. Since the second input toNAND gate 56 is connected to the collector of transistor 60, this inputis held low during the time that transistor 60 is in the conductivestate, thereby maintaining the output of NAND gate 56 high. The highoutput from NAND gate 56 on line 61, which output is inverted toinverter 62, and fed to NAND gate 63 to activate the calibrate" mode ofoperation. It is to be noted that transistor 60 will only remainconductive for a time period determined by the time constant of the R-Cnetwork formed by capacitor 58 and resistors 59 and 64. This timeconstant is typically chosen so as to provide a calibrate mode havingapproximately a half-second duration. The low output from inverter 62 online 66 acts as an inhibiting signal for NAND gate 63 and prevents thepassage of distance information through gate 63 to distance counter 81during the calibrate mode of operation.

Let us first see how the calibration of the distance circuits isaccomplished. The first negative going transition of the B receiveroutput after the calibrate switch 54 is pressed (see line B of FIG. 6A)triggers the calibrate pulse line E to HIGH," as shown in line E of FIG.6A, this end result being achieved by virtue of the gating circuitryincluding NAND gate 71 and RS flipflop 72, NAND gate 71 receiving the 8"receiver output and an enabling signal for the distance calibratefunction from inverter 73. The calibrate pulse on line E provides anenabling signal to NAND gate 74. NAND gate 74 is further enabled by theoutput of NAND gate 56, which as previously described provides anappropriate control signal throughout the calibrate mode. With gate 74thus enabled, the output of calibration oscillator 77 is fed throughgate 74 and thence through gate 75 to drive distance counter 81.Calibration oscillator 77 preferably has a frequency of l mI-Iz tofacilitate calibration.

A signal in accordance with the output count of distance counter 81 isfed to comparator 79 where this count is compared with the count set ondistance switch 52 which is fed to the comparator throughdistance-left/right selector 80. When the count of the output ofdistance counter 81 reaches the count set on distance switch 52, acalibrate reset pulse is generated by the comparator and fed on line Gto the reset input of flipflop 72, thus causing the calibrate pulse lineB to go low. When line E goes low, it provides a clock pulse forbistable flipflop 84 which in response to this clock pulse provides aclock pulse for bistable flipflop 85. The Q output of flipflop 85provides a gating signal for NAND gate 86 which, as already noted, isreceiving an enabling signal from gate 56. The output of NAND gate 86 isinverted by means of inverter 73 to provide the distance (A/B) calibratesignal on line D. The signal on line D provides several functionsincluding the setting of selector 80 for distance calibration, theenabling of NAND gate 71 and the enabling of NAND gate 88.

The next negative transition of the B receiver output again causes thecalibrate pulse line E to go HIGH. The same sequence of events asdescribed above occurs, except that the HIGH to LOW transition of thecalibrate pulse line E triggers flipflop 84, the Q output of which isinverted in inverter and fed through gate 88. A reset pulse is thusgenerated on line F, this reset pulse being fed to the divider 49 forthe A receiver output (see FIG. 4), resetting this divider to zero. Thisreset signal which is only fed to the divider for the A receiver causesthis divider output to be delayed in time from the negative transitionof the 8" receiver divider output by an amount equal in microseconds tothe number set into the distance switch 52. For the example shown inFIG. 5, this delay which is indicated in FIG. 6A on line B thereof asT,," is 3456 microseconds.

The A" receiver reset signal on line F is provided in the followingmanner: The negative going transition of the calibrate pulse on line Eprovides a clock pulse to flipflop 84. The 0" output of flipflop 84 isdifferentiated in R-C circuit 89 and fed through inverter 90 as a drivesignal for NAND gate 88. As already noted, NAND gate 88 is enabled bythe signal on line D and therefore passes this reset signal through toline F.

The calibration of the left-right receivers is then achieved in thefollowing manner: The negative transition of the 8" receiver output,designated in FIG. 6A as B, initiates the calibrate sequence, causing acalibrate pulse to be generated on line E. However, the next HIGH to LOWtransition of the calibrate pulse line E causes the distance calibrationgate signal on line D to go LOW, and the left-right calibration gate online J to go HIGH. This end result is implemented by means of the toggleaction of flipflops 84 and 85, flipflop 85 operating at one half thefrequency of flipflop 84. Thus, the negative transition of the calibratepulse on the line E drives flipflop 84 to provide a drive pulse forflipflop 85. The Q output of this flipflop goes LOW and the 6" outputHIGH.

Thus, in this manner, gate 86 is inhibited and gate 92 enabled. Theoutput of gate 92 is inverted by inverter 94 and fed to line J toprovide the left-right calibration enabling signal. The signal on line Joperates to cause selector 80 to feed the output of left-right switch 53to comparator 79. This signal also provides an enable signal for leftand right receiver reset gates 94 and 95, as well as providing enablingsignals for NAND gates 97 and 98.

Let us assume first that separate receivers are to be used for each ofthe distance and left and right signals (i.e., there is no dual use ofone receiver for both one of the left-right and one of the distancesignals). Under such conditions, the signal on line .I provides anenabling signal to both gates 97 and 98. However, to be actuated, gate98 requires a further enabling signal from gate 99, indicating that oneof the steering and distance receivers is not to be combined. Gate 99operates in response to logical gate 100 which is manually set in thisinstance to provide an output signal indicating that a combination ofreceiver signals is not to be utilized. Gate 98 thus is enabled suchthat the first signal from the LEFT receiver on line H occurring afterthe commencement of the left-right calibrate gate (line J) will passthrough gate 98 to provide a set signal for calibrate flipflop 72. Theoutput of calibrate flipflop 72 (line E) thus goes HIGH in response tothe negative going transition of the LEFT receiver.

The sequence of operation that follows is the same as described for thedistance receivers except that the output of distance counter 81 is nowcompared with the output of left-right switch 53 in comparator 79 ratherthan the output of distance switch 52, such operation being implementedby virtue of the presence of the control signal on line .I which is fedto selector 80. Thus, the output of comparator 79 resets calibrateflipflop 72 when the count set on switch 53 is reached. The calibratesignal on line E thus is given a pulse width corresponding to thiscount.

The HIGH to LOW transition of the output of flipflop 72 triggersflipflop 84 which is inverted in inverter 90. The output ofinverter 90in turn provides a reset signal through gate 95 which appears on line F,and is used to reset the divider which receives the output of the RIGHTreceiver. This causes the output of the RIGHT receiver divider to bedelayed in time from that of the LEFT receiver divider output by anamount equal in microseconds to the setting of switch 53 (in theillustrative example 5030 microseconds). This is noted under line G ofFIG. 6A as T, 5000 u.

The next HIGH to LOW transition of the calibrate pulse line E causes theleft-right calibrate gate signal to go LOW and the A/B calibrate signalto go HIGH, and so on for the duration of the calibrate cycle(approximately one-half second). In this manner, the distance andsteering signals are alternately calibrated during the calibrate mode ofoperation.

Where the same receiver is utilized dually to provide both one of thedistance and one of the steering signals,

for example both R" and A," the calibration sequence must be somewhataltered since it will immediately be apparent that the same receiveroutput divider cannot be calibrated to permform both functions. Nochange is necessary in the A-B distance calibration, the A receiverbeing calibrated as just described to the number set in the distanceswitch. However, during the L-R calibrate sequence, the R receiveroutput negative transition is used to set the calibrate pulse line HIGHinstead of the L" receiver output. This operation is described in thewaveforms shown in FIG. 68 (lines K-O). Control operates to feed theoutput of the RIGHT receiver as utilized for the A function through thegating circuitry to provide a reset signal for the left receiver throughgate 94 on line N, in the same general manner as previously described.It is further to be noted that the number set into left-right switch 53in this instance is found by subtracting the computed number from10,000. That is to say, for the illustrative example of FIG. 5, switch53 would be set to 4970.

Referring now to FIGS. 7 and 8, the circuitry for performing thedistance measurement in the system of the invention and waveformsrelevant thereto are respectively illustrated. As already described inconnection with FIGS. 1 and 2, two radio stations, A and B, are used forthe distance measurements, these stations being chosen so that thedirection of travel is away from station 3" and towards station A."Further, a number representing the difference in microseconds betweenthe two signals as received at the point origin and as received at thepoint of destination is fed into the system by appropriately resettingone of the receivers dividers during a calibration mode of operation asdescribed above. The output of the 8" receiver shown on line B" of FIG.8 is provided as a set input to flipflop 24, while the A receiver outputis fed to the reset terminal of this flipflop. Flipflop 24 has an outputas indicated on line C, this output representing the distance to go tothe destination point.

Referring now to FIG. 10a, typical outputs at times T T T T and T forvarious positions at and between the origin and destination points areillustrated. These are shown for the situation where the calibration atthe origin is 3456 microseconds. As can be seen, the output of distanceflipflop 24 gradually decreases and finally reaches zero when thedestination point is reached at time T,,.

In order for the system to provide distance readings to the destinationin miles, it is necessary to enter into the system the straight linedistance between the starting point and the destination. This is done byadjusting the frequency of variable frequency oscillator 103 by means ofknob 103a so that the distance readout 26, which may be on appropriatedigital readout device, reads this distance when the vehicle is at theorigin. The output of variable freqeuncy oscillator 103 is fed to NANDgate 63. Also fed to this gate is the output of distance flipflop 24(line C) and an enable signal from timing generator 105 (line D). Timinggenerator 105 is synchronized with a negative transition of the outputof flipflop 24 and operates to periodically provide samplings of thedistance measurements for distance readout 26. The details of the timinggenerator need not to explained, as this may comprise any standarddigital timing circuit as is well known in the art. Thus, periodicallygate 63 will have a burst of output pulses at a frequency of oscillator103, and for a time period determined by the pulse width of the outputof distance flipflop 24. As already explained in connection with FIG.with the description of the calibration function, gate 63 also receivesa signal which inhibits its operation during the calibrate mode so thatno distance measurements can be made during that time.

Distance counter 81 also receives a reset signal from timing generator105 through NAND gate 110 which resets the counter to zero before eachdistance measurement is made. In a typical operating embodiment of theinvention, distance measurements are made on every 200th distanceflipflop output cycle as shown in FIG. 8. Referring particularly now toFIG. 8, with the distance flipflop output cycle marked 1 in line C, theoutput of timing generator 105 goes HIGH when the output of the distanceflipflop 24 goes LOW. At the same time, the timing generator sends areset pulse (line E) through gate 110, which resets distance counter 81to zero. When the distance flipflop cycle 2 goes HIGH (line C), theoutput of variable frequency oscillator 103 is fed through gate 63 (lineF) and gate 75 (line G) to distance counter 81 which counts the totalnumber of clock pulses that occur during the period that the distanceflipflop output is HIGH. The negative transition of distance flipflopoutput cycle 2 terminates the count of the distance counter and the sametime causes the output (line D) of timing generator 105 to go LOW,thereby inhibiting gate 63. Gate 63 is inhibited in this fashion byvirtue of the programming set into timing generator 105 until thenegative transition of the distance flipflop output cycle 199 (line C),at which time the output of the timing generator goes HIGH and themeasurement cycle is repeated. During the time that the output of timinggenerator 105 to gate 63 is LOW inhibiting this gate, the distancemeasurement is displayed on distance readout 26. In this manner, areadout is repetitively provided in terms of miles to the destination inaccordance with the pulse width of the output of distance flipflop 24.

Referring now to FIGS. 9, 9A, B, 10C and 11, the operation of thesteering control in the system of the invention will now be described.Referring particularly to FIGS. 9 and 9A, the divided outputs of the Land R receivers of FIG. 3'are fed to the set and reset inputs offlipflop 28. Thus, the output of flipflop 28 is a rectangular wave, theoutput of which goes HIGH in response to the L" receiver and LOW inresponse to the R receiver output. The output flipflop 28v is fed tosumming resistors 121 of steering computer 29. The output of distanceflipflop 24 is fed through the calibration circuit including transistors113, 116 and 117 and resistors 118, 119 and 122, the summing resistors121 for summation with the output of flipflop 28. The output of flipflop24 is fed through resistor 111 to transistor 113 and through resistor112 to transistor 117. The output of transistor 113 is fed throughresistor 115 as a drive signal for transistor 116. Bias is provided fortransistor 116 from voltage source 120 through resistor 114. Thecollectors of transistors 116 and 117 are interconnected by resistors118 and 119.

Voltage source 120 provides a voltage E (as shown in FIG. 9A) to theemitter of transistor 116. Equal resistance resistors 130 and 131 form avoltage divider across voltage source 120 and develop reference voltageE; which is equal to E/2. This reference voltage is fed to theinterconnection between equal resistance resistors 118 and 119 as wellas differential amplifier 127 and Left-Right indicator 30. The oppositeends of calibration potentiometer 122 are connected across resistors 118and 119, the arm of this potentiometer being connected to one of summingresistors 121. The summed output of resistors 121 is fed to differentialamplifier 127.

It will be readily apparent as indicated in FIG. 9A that the signal atthe collector of transistor 116 (point A) and the signal at thecollector of transistor 117 (point B) are rectangular waves which are inphase opposition in an edge to edge mirror image relationship. Thesewaves as shown in FIG. 9A run above and below E respectively.

Consider the condition where the negative transition of the R receiveroutput is calibrated to occur 5,000 microseconds after the negativetransition of the L" receiver output. The L receiver output sets theoutput of flipflop 28 HIGH, and the R receiver output resets it LOW.With the outputs of all of the receivers having a repetition rate of lOOH, the output of flipflop 28 will be a Hz symmetrical square wave. Theaverage DC value of this square wave will be E/2 where E is itspeak-to-peak voltage. With the reference voltage E equal to BIZ and ifthe signal fed from distance flipflop 24 to transistor 113 is equal tozero, the output of summing resistors 12] will be equal to E so thatindicator needle 30 will be centered.

However, if the output of flipflop 28 becomes unsymmeteical because of arelative time shift between the L and R receiver outputs, the needle ofindicator 30 will deflect to the left or right ofits zero centerposition, depending upon whether the result in average DC is less thanor greater than E To illustrate how the device typically operates,attention is now directed to FIGS. 10Al0C and FIG. 11. Referringparticularly to FIG. 11, lines It represent loci of constant phaserelationship between signals received from the L and R transmitters.Line m represents a flight path between origin 1 and destination 1" thatis at all times parallel to the lines of constqnt phase so that therelative phase between the L and R receiver outputs remains constantthroughout this trip. Thus, with the system calibrated to make theoutput of flipflop 28 symmetrical before the trip is commenced, theplane would be flown to maintain this condition throughout the trip. Itis to be noted that for this type of operation, it would be necessary toadjust potentiometer 122 for a balanced reading on indicator 30 beforestarting the trip.

Let us now consider a more typical trip between origin No. anddestination No. 2 as indicated by line n in FIG. 11 which cuts acrosslines of constant phase. For the purpose of this discussion it isassumed that the L" and R" transmitters are far enough away and the tripsufficiently short that the lines of constant phase can be considered tobe parallel straight lines. From the diagram of FIG. 11, it can be seenthat the signal from the L station arrives earlier at destination No. 2than at origin No. 2 by the amount T which can be described as follows;

T A distance/V where A distance is as indicated in FIG. 11 and V is thevelocity of propagation of this signal from the L" transmitter.Similarly, the signal from the R transmitter arrives later atdestination No. 2 than at origin No. 2 by the amount T which can berepresented as follows:

T A distance/V where V is the velocity of propagation of the signal fromthe R transmitter. Since it is the L receiver output that is setting theoutput of flipflop 2.8 and the R receiver output that is resetting it,it is apparent that the HIGH state of the output of the flipflop willbecome progressively longer as the flight path from origin No. 2 todestination No. 2 is travelled. This condition is shown in FIG. 10C forvarious positions as indicated in FIG. 11 along the flight path n. Asalready noted in connection with FIG. 10A, it can be seen that theoutput of distance flipflop 24 progressively decreases in width from theorigin to destination, this decrease being linear as shown in FIG. 2.Thus, by adding a portion of a signal in accordance with the output ofdistance flipflop 24, the ratio of this portion being determined by theinitial setting of potentiometer 122, it can be seen that a compensationsignal T is provided as graphically indicated in FIG. 2 which willprovide a zero output from indicator 30 as long as we remain on theproper track. It is to be noted, of course, that in order to provide adirect track to the destination, it is important that prior to departurefrom the point of origin potentiometer 122 be set so that the output ofamplifier 127 is zero so as to provide a zero indication on indicator30.

It should be immediately apparent that since the compensating signalfrom flipflop 24 goes to zero at the destination, the output of flipflop28 must go to 5,000 microseconds (symmetrical square wave) at the sametime in order for the indicator 30 to be centered at the destination. Asalready explained in connection with FIGS. and 6, the system must ofcourse initially be calibrated before departure from the origin so thatthe output of flipflop 28 is shorter than 5,000 microseconds by thedifference in travel time relationship between the signals from the Leftand Right transmitters as received at the origin and as received at thedestination. With the assumptions made in this discussion, that is, thatthe lines of constant phase k are parallel straight lines, the flightpath to maintain indicator 30 centered will be a straight line betweenthe origin and destination. However, since the lines of constant phasemay have some curvature, the actual flight path to maintain thiscondition may be somewhat curved but will always pass through the pointof origin and the point of destination.

Referring now to FIG. B and FIG. 11, another hypothetical trip betweenorigin No. 3 and destination No. 3 as represented by track line 0 isillustrated. In this trip, the flight path is towards the R" trnsmitterand away from the L transmitter. Here, as can be seen, the output offlipflop 28 decreases in width by the amount T during the voyage. Sinceit is necessary for the output of flipflop 28 to be equal to 5,000microseconds at the destination when the contribution of the distanceflipflop 24 is zero, it is necessary to calibrate at the outset suchthat the output of flipflop 28 at the origin is greater than 5,000microseconds by the expected change in phase during the voyage, at theorigin.

Indicator 30 is then centered by substracting a sufficient amount of thedistance flipflop output from the L-R flipflop output so that thecomposite signal is equal to that ofa symmetrical square wave. Oncecentered as described above, the needle of indicator 30 will remaincentered as long as the indicated flight path between the origin anddestination is followed.

The system of this invention thus provides means for navigating betweenan origin and destination by utilizing signals from regular radiotransmitters or those established for use in other types of systems. Nodirective antennas or complicated computing equipment is required on thevehicle, and steering information as well as information as to distanceto the destination is continually provided in easily usable form.

We claim:

1. In a radio navigation system for navigating a vehicle along a trackline between an origin and a destination point utilizing a plurality ofradio signals, including signals transmitted from points ahead of andbehind said track line and to the left and right of said track line,

means for converting said signals to a common frequency,

means for generating a distance signal in accordance with the phaserelationship between a first pair of said common frequency signalsrepresenting signals transmitted respectively from ahead of and behindsaid track line, said distance signal representing distance to go to thedestination,

means for calibrating said distance signal at the origin so that itrepresents the change in the travel time difference between said firstpair of signals as received at the origin and as received at thedestination,

means for generating a left-right signal in accordance with the phaserelationship between a second pair of said common frequency signalsrepresenting signals transmitted respectively from the left and right ofsaid track line, and

means for generating a steering signal indicating any deviation fromsaid track line in response to the algebraic sum of signals inaccordance with said Left- Right signal and said distance signal.

2. The system of claim 1 wherein said means for generating a steeringsignal comprises a steering computer for computing the vehicle positionrelative to said track line and an indicator responsive to the output ofsaid computer for indicating any deviations to the left or to the rightof said line.

3. The system of claim 2 wherein said steering computer includes acalibration potentiometer for adjusting the distance signal when thevehicle is at the origin to produce a zero deviation indication on' saidindicator.

4. The system of claim 1 and further including a digital distancereadout for providing a continual indication of distance to go to thedestination in response to said distance signal.

5. The system of claim 1 and including means for calibrating saidLeft-Right signal at the origin to represent the change in the traveltime difference between said second pair of signals as received at theorigin and as received at the destination.

6. The system of claim 3 wherein said steering computer includes a DCreference voltage source for establishing a reference voltage for saidindicator and said steering computer, said computer providing a DCoutput to said indicator above said reference voltage when the vehicleis on one side of the track line and below said reference voltage whenthe vehicle is on the other side of the track line.

7. A method for providing navigation control signals for guiding avehicle on a track line between an origin and destination point byutilizing a plurality of radio signals including a first pair of signalstransmitted from points ahead of and behind said track linerespectively, and a second pair of signals to the left and right thereofrespectively, comprising the steps of:

setting into a distance measuring circuit a calibration factor inaccordance with the change in travel time difference between said firstpair of signals as received at the origin and destination points,

setting into a Left-Right measuring circuit a calibration factor inaccordance with the change in travel time difference between said secondpair of signals as received at the origin and destination points,

feeding said first pair of signals to said distance measuring circuitand deriving a signal in accordance with the time relationshiptherebetween representing distance to go to the destination,

feeding said second pair of signals to said Left-Right measuringcircuit, and

summing the output of said Left-Right measuring circuit with a signal inaccordance with the output of said distance measuring signal to providea signal for steering the vehicle along said track line.

8. The method of claim 7 wherein the distance calibration factor is suchthat the output of the distance measuring circuit goes to zero when thevehicle arrives at the destination, and the Left-Right calibrationfactor is such that the output of the Left-Right measuring circuit goesto a value representing zero deviation from the track line when thevehicle arrives at the destination.

9. The method of claim 7 and further including the step of digitallyreading out the output of said distance measuring circuit to provide acontinuous readout signal in accordance with miles to go to thedestination point.

10. In a radio navigation system for navigating along a track linebetween an origin and destination point by utilizing a first pair ofsignals for deriving distance information received from a firsttransmitter ahead of the track line and a second transmitter behind thetrack line, and a second pair of signals for deriving steeringinformation received from transmitters to the left and right of thetrack line comprising:

receiver means for receiving said signals;

divider means for dividing said signals down to a common frequency;distance phase detector means for generating a distance signal inaccordance with the time relationship between the pair of signalsreceived from the transmitters ahead of and behind the track line;

left-right phase detector means for generating a signal in accordancewith the time relationship between the signals received from thetransmitters to the left and the right of the track line;

digital distance computer means for deriving a signal in accordance withdistance to the destination in accordance with the output of saiddistance phase detector;

calibration control means for calibrating said digital distance computermeans so that the output thereof represents the distance between theorigin and destination when the vehicle is at the origin and is zerowhen the vehicle reaches the destination;

distance readout means for providing a readout in accordance with theoutput of said digital distance computer, and

steering means including steering computer means for deriving a signalindicative of the position of the vehicle relative to the track linebetween the origin and destination, said steering computer meansoperating in response to the output of said left-right phase detectorand said distance phase detector and including means for summing asignal proportional to a preselected portion of the output of thedistance phase detector with the output of the leftright phase detector,the distance phase detector output providing a compensating signal whichdecreases linearly from a maximum value at the origin to zero at thedestination.

11. The system of claim 10 wherein said steering means includes steeringindicator means for indicating any deviation from said track linebetween origin and destination, said steering indicator means operatingin response to the output of said steering computer means.

12. The system of claim 11 wherein said steering computer means includesmeans for adjusting the output of the distance phase detector at theorigin to provide a zero" indication on said steering indicator means.

13'. The system of claim 10 wherein said calibration control meansincludes means for calibrating the distance computer means at the originso that the distance signal represents the change in the travel timedifference between the signals from transmitters ahead of and behind thetrack line as received at the origin and as received at the destination.

14. The system of claim 12 wherein said steering computer means includesa DC reference voltage source for establishing a reference voltage forsaid indicator and said steering computer means, said computer meansproviding a DC output to said indicator above said reference voltagewhen the vehicle is on one side of the track line and below saidreference voltage when the vehicle is on the other side of the trackline.

15. The system of claim 10 wherein said calibration control meansincludes means for resetting the divider means for one signal of each ofsaid pairs of received signals.

' 16. The system of claim 15 wherein said means for resetting thedivider means includes distance switch means for manually setting asignal representing the distance to be traversed between the origin anddestination and left-right switch means for manually setting a signalrepresenting the track line orientation and control means responsive tothe settings on said switch means for generating the divider resetsignals.

t fi

1. In a radio navigation system for navigating a vehicle along a trackline between an origin and a destination point utilizing a plurality ofradio signals, including signals transmitted from points ahead of andbehind said track line and to the left and right of said track line,means for converting said signals to a common frequency, means forgenerating a distance signal in accordance with the phase relationshipbetween a first pair of said common frequency signals representingsignals transmitted respectively from ahead of and behind said trackline, said distance signal representing distance to go to thedestination, means for calibrating said distance signal at the origin sothat it represents the change in the travel time difference between saidfirst pair of signals as received at the origin and as received at thedestination, means for generating a left-right signal in accordance withthe phase relationship between a second pair of said common frequencysignals representing signals transmitted respectively from the left andright of said track line, and means for generating a steering signalindicating any deviation from said track line in response to thealgebraic sum of signals in accordance with said Left-Right signal andsaid distance signal.
 2. The system of claim 1 wherein said means forgenerating a steering signal comprises a steering computer for computingthe vehicle position relative to said track line and an indicatorresponsive to the output of said computer for indicating any deviationsto the left or to the right of said line.
 3. The system of claim 2wherein said steering computer includes a calibration potentiometer foradjusting the distance signal when the vehicle is at the origin toproduce a zero deviation indication on said indIcator.
 4. The system ofclaim 1 and further including a digital distance readout for providing acontinual indication of distance to go to the destination in response tosaid distance signal.
 5. The system of claim 1 and including means forcalibrating said Left-Right signal at the origin to represent the changein the travel time difference between said second pair of signals asreceived at the origin and as received at the destination.
 6. The systemof claim 3 wherein said steering computer includes a DC referencevoltage source for establishing a reference voltage for said indicatorand said steering computer, said computer providing a DC output to saidindicator above said reference voltage when the vehicle is on one sideof the track line and below said reference voltage when the vehicle ison the other side of the track line.
 7. A method for providingnavigation control signals for guiding a vehicle on a track line betweenan origin and destination point by utilizing a plurality of radiosignals including a first pair of signals transmitted from points aheadof and behind said track line respectively, and a second pair of signalsto the left and right thereof respectively, comprising the steps of:setting into a distance measuring circuit a calibration factor inaccordance with the change in travel time difference between said firstpair of signals as received at the origin and destination points,setting into a Left-Right measuring circuit a calibration factor inaccordance with the change in travel time difference between said secondpair of signals as received at the origin and destination points,feeding said first pair of signals to said distance measuring circuitand deriving a signal in accordance with the time relationshiptherebetween representing distance to go to the destination, feedingsaid second pair of signals to said Left-Right measuring circuit, andsumming the output of said Left-Right measuring circuit with a signal inaccordance with the output of said distance measuring signal to providea signal for steering the vehicle along said track line.
 8. The methodof claim 7 wherein the distance calibration factor is such that theoutput of the distance measuring circuit goes to zero when the vehiclearrives at the destination, and the Left-Right calibration factor issuch that the output of the Left-Right measuring circuit goes to a valuerepresenting zero deviation from the track line when the vehicle arrivesat the destination.
 9. The method of claim 7 and further including thestep of digitally reading out the output of said distance measuringcircuit to provide a continuous readout signal in accordance with milesto go to the destination point.
 10. In a radio navigation system fornavigating along a track line between an origin and destination point byutilizing a first pair of signals for deriving distance informationreceived from a first transmitter ahead of the track line and a secondtransmitter behind the track line, and a second pair of signals forderiving steering information received from transmitters to the left andright of the track line comprising: receiver means for receiving saidsignals; divider means for dividing said signals down to a commonfrequency; distance phase detector means for generating a distancesignal in accordance with the time relationship between the pair ofsignals received from the transmitters ahead of and behind the trackline; left-right phase detector means for generating a signal inaccordance with the time relationship between the signals received fromthe transmitters to the left and the right of the track line; digitaldistance computer means for deriving a signal in accordance withdistance to the destination in accordance with the output of saiddistance phase detector; calibration control means for calibrating saiddigital distance computer means so that the output thereof representsthe distance beTween the origin and destination when the vehicle is atthe origin and is zero when the vehicle reaches the destination;distance readout means for providing a readout in accordance with theoutput of said digital distance computer, and steering means includingsteering computer means for deriving a signal indicative of the positionof the vehicle relative to the track line between the origin anddestination, said steering computer means operating in response to theoutput of said left-right phase detector and said distance phasedetector and including means for summing a signal proportional to apreselected portion of the output of the distance phase detector withthe output of the left-right phase detector, the distance phase detectoroutput providing a compensating signal which decreases linearly from amaximum value at the origin to zero at the destination.
 11. The systemof claim 10 wherein said steering means includes steering indicatormeans for indicating any deviation from said track line between originand destination, said steering indicator means operating in response tothe output of said steering computer means.
 12. The system of claim 11wherein said steering computer means includes means for adjusting theoutput of the distance phase detector at the origin to provide a''''zero'''' indication on said steering indicator means.
 13. The systemof claim 10 wherein said calibration control means includes means forcalibrating the distance computer means at the origin so that thedistance signal represents the change in the travel time differencebetween the signals from transmitters ahead of and behind the track lineas received at the origin and as received at the destination.
 14. Thesystem of claim 12 wherein said steering computer means includes a DCreference voltage source for establishing a reference voltage for saidindicator and said steering computer means, said computer meansproviding a DC output to said indicator above said reference voltagewhen the vehicle is on one side of the track line and below saidreference voltage when the vehicle is on the other side of the trackline.
 15. The system of claim 10 wherein said calibration control meansincludes means for resetting the divider means for one signal of each ofsaid pairs of received signals.
 16. The system of claim 15 wherein saidmeans for resetting the divider means includes distance switch means formanually setting a signal representing the distance to be traversedbetween the origin and destination and left-right switch means formanually setting a signal representing the track line orientation andcontrol means responsive to the settings on said switch means forgenerating the divider reset signals.